Volatile phosphoric esters are now widely used as pesticides, particularly as evaporator insecticides. Their wide use is due principally to their rapid action as vapors and the absence of any accumulation of the compounds in living tissue as a result of their rapid hydrolysis in situ.
This last characteristic, which gives a net advantage over "chlorinated" pesticides, is on the other hand, a serious disadvantage: the sensitivity of certain phosphoric acid esters to humidity, even just that of the atmosphere, is such that decomposition takes place in the evaporator before evaporation and before they are able to act on the pest organisms. The esters particularly susceptible to this are those containing low alkyl groups such as methyl, ethyl, propyl or isopropyl attached to the phosphoric anion. On contact with molecules of water, at least partial decomposition of the esters takes place by protonisation, i.e. by replacement of a low alkyl group by hydrogen.
Among the sensitive phosphoric esters, special mention should be made of 0-2,2-dichlorovinyl-0,0-dimethyl phosphate, better known by the common name of DICHLORVOS or DDVP, the use of which in permanent insecticidal devices, so-called evaporators, has risen very greatly during recent years.
Various methods of stabilisation of the sensitive phosphoric esters in the evaporator have already been suggested to limit the decomposition of there phosphoric esters, but they are generally toxic such as phenols, amines or low nitrogen heterocyclics; another class of useful stabilizers includes azoic and hydrazonic compounds, but these possess a strong colouring ability which does not always permit them to be used. The use of anhydrides or epoxides has also been suggested, but it is known that these compounds act by fixation either of a molecule of water or of a molecule of free acid: it is clear that this process is stoichiome trically limited and that stabilisation ceases when all the stabiliser has reacted. This leads to the necessity of using substantial proportions of these stabilisers, which is not economic.